The present invention relates to a method, apparatus, and computer program for processing color digital images to reduce noise.
The image structure of color digital images can be thought of as made up of two components: a signal component and a noise component. In many digital imaging applications it is desirable to amplify the signal component while in the same instance reduce the noise component. The difficulty of achieving both these goals for a color digital image processing application simultaneously lies in the inherent differentiability of the signal and noise components. No technology at present achieves a perfect decomposition of image data into these two components. Thus current image processing noise reduction algorithms result in either some loss of the signal component or not enough noise component removal or both. It has been recognized in the image photographic field that the color of different real world objects can be used to advantage with regard to signal and noise processing.
In U.S. Pat. No. 5,682,443 Gough and MacDonald disclose a method of processing color digital images for the purposes of spatial sharpness characteristic enhancement. In this disclosure, a method of unsharp masking is discussed which separates an original color digital image into two parts based on the spatial frequency content of the original color digital image. The difference between the original pixel values and a low spatial frequency component of the original pixel values forms a fringe component, or high spatial frequency component. Gough and MacDonald teach a method of modifying the fringe component based on the color of either the low spatial frequency component or the original pixel values. Their method also discloses that the preferred method of implementing this feature uses a continuous mathematical function of color. The method disclosed by Gough and MacDonald takes advantage of color as an image characteristic for enhancing the spatial detail. However, the unsharp masking procedure employed by Gough and MacDonald is designed to vary the amplification of the signal component and is not effective for the purposes of reducing of the noise component.
In U.S. Pat. No. 4,689,666 Hatanaka discloses a method of using the color characteristics of a color digital image for the purposes of reducing the noise component of the color digital image. In the method disclosed, Hatanaka describes a process of extracting color data for each picture element of the image, discriminating regions of the color digital image exhibiting a specific color on the basis of the extracted color data, and subjecting the image signal to spatial image processing for elimination of noise under different processing conditions for regions exhibiting the specific color and the remaining regions not exhibiting the specific color. Thus the method taught by Hatanaka has as a fundamental step the segmentation, or discrimination, of each pixel as belonging to the specific color or not belonging to the specific color. The step color discrimination can lead to unevenness in the processed images due to the on/off nature of the color identification process.
There is thus a need for a noise reduction algorithm which uses the color characteristic of a color digital image to advantage the process of noise reduction in a smooth and even manner without producing switching artifacts typical of segmentation based decision based methods.
The need is met according to the present invention by providing a digital image processing method for reducing the noise of a color digital image having pixel values, that includes the steps of: identifying one or more color regions in color space; identifying a pixel of interest in the color digital image; calculating a color weighting factor for the pixel of interest, the color weighting factor being a near continuous function of the one or more color regions; using the color weighting factor and the values of pixels sampled from a local neighborhood of pixels about the pixel of interest to calculate a noise reduced pixel value; replacing the original value of the pixel of interest with the noise reduced pixel value; and repeating the steps for other pixels in the color digital image.
The present invention has the advantage that noise reduction is applied to preferentially treat different regions of color space in a continuous fashion so that spatial artifacts are avoided. It has the further advantage that different color regions, such as skin, sky, and grass can be treated differently to obtain a pleasing image.